Chemistry Reference
In-Depth Information
The nanoscale size turns out to be critical for some structural characteristic
of silicon carbide nanowires, such as the crystallographic orientation, strongly
dependent on the nanowire diameter.
The most common growth direction for VLS growth mechanism in SiC literature
is the nanowire axis parallel to
<
111
>
crystallographic orientation, but in some
cases also
<
100
>
direction has been observed (Zhou et al.
1999
). This is strongly
dependent from the diameter, since crystallographic orientation of the nanowire is
driven by the minimization of the total free energy. The contribution to the total
free energy are the bulk energy of the nanowire, the surface energy of the interface
between liquid catalyst and nanowire and finally the nanowire surface in vacuum
(similar to that in gas atmosphere). With small nanowire diameters the surface energy
of the NW in vacuum is dominating but, for bigger diameters, the contribution of
the surface free energy of the interface between the semiconductor and the metal
catalyst becomes predominant (Wagner
1970
; Fortuna and Li
2010
).
5.2.2
Vapour-Solid-Solid Growth Mechanism
In many cases nanowires growth was observed at temperatures far below the eutectic
point of the semiconductor-metal system (Persson et al.
2004
). Even if the melting
point of a small body is smaller than the corresponding bulk (Buffat and Borel
1976
),
the temperature used and the size of the nanoparticles exclude the possibility for these
particles to melt.
In some circumstances (Baker
1989
) this phenomenon was explained with a
growth mechanism very similar to the VLS, but presuming the solid state diffu-
sion through the solid catalyst nanoparticle. In general, when an axial growth is
nourished from gas phase and catalysed by a solid particle the growth mechanism is
called vapour-solid-solid.
It is hard to find a strict borderline between VLS and VSS, since the catalyst
nanoparticle act as a nucleation point in both cases and there are many examples in
which the structure of the nanowires seems not to be affected by the catalyst phase
(Kolasinski
2006
).
5.2.3
Vapour-Solid Growth Mechanism
The presence of a catalytic specie isn't always necessary, since many other factors
can induce an axial growth. The nucleation probability P
N
on the surface of a whisker
is related to thermodynamic variables and to crystallographic parameters:
P
N
=
B
exp
πγ
2
k
2
T
2
ln
σ
−
(5.2)
(after Dai et al. (
2003
) and Sears (
1955
))
